cls/rbd: update last read in group::snap_list

cls/rbd: update last read in group::snap_list

Problem:
`rbd group snap ls` shows 1025 records after creating 65 snaps
with `rbd group snap create`

$ for i in {1..65}; do rbd group snap create test_group@group_snap$i; done
$ rbd group snap ls test_group | wc -l
1025

Solution:
update last_read after getting RBD_MAX_KEYS_READ with cls_cxx_map_get_vals.

Fixes: https://tracker.ceph.com/issues/57066
Signed-off-by: Prasanna Kumar Kalever <prasanna.kalever@redhat.com>